a) Field of the Invention
The present invention relates to a process of fabricating a magnetic hard disk substrate having an excellent surface flatness.
b) Description of the Prior Art
There has been a general trend toward a higher density and larger capacity of a magnetic hard disk device as a storage device of a computer and the like and it has, therefore, been an important technical aspect to finish a magnetic hard disk substrate used for the device to a desired surface roughness and flatness. Particularly, there has been achieved a conspicuous development in the reduction of levitation based on an improvement in the head side part for the realization of a high density recording and thereby, the gap between a head and a medium has been narrowed and in company with this, further improvement in not only the surface flatness of a magnetic hard disk substrate but also the surface roughness thereof has strongly been demanded in order to prevent a head crash.
It is common that a magnetic hard disk substrate is fabricated by a process comprising the steps of: obtaining a blank by blanking a rolled coil of aluminum or aluminum alloy (hereinafter referred to as simply aluminum); grinding a surface of the blank so as to have surface damage, waviness and the like removed to attain a substrate; effecting Ni-P plating on the surface; performing finish polishing on the plated surface; thereafter performing magnetic sputtering on the polished surface to form a magnetic coating layer; and applying an overcoat on the layer to obtain a sputtering medium. In order to control the surface roughness and flatness within a predetermined range in the process, in the grinding step applied prior to the Ni-P plating step, the magnetic hard disk substrate (as a blank) is subjected to rough grinding by a grinding stone with a grain size of the order of #800 in a double-surface grinding machine, which is followed by finish grinding by a grinding stone with a grain size of the order of #3000 in the grinding machine. This kind of grinding is called a G--G (Grinding-Grinding) process, for this grinding step comprises two sub-steps of grinding.
In a conventional G--G process, however, since a first step of rough grinding has been applied on plural Aluminum blanks in one operation by a grinding stone or the like, the removal of stock by grinding is limited and a stock removal large enough to effect grinding off a bow, waviness or the like has been hard to achieve. Therefore, in a second step of grinding, which is performed following the rough grinding in a similar way thereto, the removal of stock is obliged to be larger, even with which it is still difficult to reduce the substrate-to-substrate thickness variation. Instead of the G--G process, there is also a well-known process, which is a so-called T-G process, in which in a first step, turning is performed and in a second step, grinding is performed. According to the T-G process, as a first step, turning by use of a precision lathe is performed and as a second step following the first step, grinding is performed, so that while a bow, waviness and the like of a magnetic hard disk substrate are eliminated, the magnitude of a grinding stress is dispersed by an uneven stock removal, which is, in turn, caused by a substrate-to-substrate thickness variation, and a deteriorated surface flatness of the substrate results, which have been causes for making severe requirements for a magnetic hard disk substrate difficult to satisfy.